Method and apparatus for modulating the radiation dose from x-ray tube

Abstract

In a method for modulating a radiation dose from an x-ray tube, and in an x-ray tube-containing apparatus, the x-ray tube has at least one variable operating parameter which, when varied, modulates the radiation dose, with a modulation speed, for x-rays produced by the x-ray tube, and the x-ray tube is operated while varying this parameter through a parameter range to generate modulation speed data, representing modulation speeds of the x-ray tube respectively for different values of the operating parameter. When an examination subject is to be irradiated with an x-ray dose from the x-ray tube, an exposure effect associated with the exposure of the subject to the radiation dose is identified, this exposure effect being dependent on modulation of the radiation dose. When exposing the subject to the radiation dose, the operating parameter is varied in advance of a time at which the exposure effect is to be achieved, dependent on the modulation speed data, so that the radiation dose is modulated to produce the desired exposure effect at the desired time.

I claim as my invention:  
     
       1. A method for modulating a radiation dose from an x-ray tube, comprising the steps of: 
       (a) providing an x-ray tube having at least one variable operating parameter which, when varied, modulates a radiation dose, with a modulation speed, for x-rays produced by said x-ray tube;  
       (b) operating said x-ray tube while varying said operating parameter through a parameter range and generating modulation speed data representing modulation speeds respectively for different values of said operating parameter;  
       (c) identifying an exposure effect, associated with exposure of a subject to said radiation dose, that is dependent on modulation of said radiation dose; and  
       (d) varying said operating parameter in advance of a time at which said exposure effect is to be achieved, according to said modulation speed data, to modulate said radiation dose while irradiating said subject with x-rays from said x-ray tube to produce said exposure effect at said time.  
     
     
       2. A method as claimed in  claim 1  comprising selecting said at least one operating parameter from the group consisting of maximum tube current, focus size and modulation index. 
     
     
       3. A method as claimed in  claim 1  wherein step (a) comprises providing an x-ray tube having a plurality of variable operating parameters which, when varied, modulate a radiation dose, with a modulation speed, for x-ray produced by said x-ray tube, and wherein step (b) comprises operating said x-ray tube while varying multiple operating parameters in said plurality of operating parameters through respective parameter ranges to generate modulation speed data representing modulation speeds respectively for different combinations of said multiple operating parameters, and wherein step (d) comprises varying said multiple operating parameters in advance of said time. 
     
     
       4. A method as claimed in  claim 3  comprising selecting said multiple operating parameters from the group consisting of maximum tube current, focus size and modulation index. 
     
     
       5. A method as claimed in  claim 1  wherein said exposure of said subject to said radiation dose in step (c) comprises rotating said x-ray tube on a gantry around said subject to expose said subject to x-rays from said x-ray tube at a plurality of successive projection angles in a computed tomography examination of said subject, and wherein step (a) comprises selecting said at least one variable operating parameter from the group consisting of maximum tube current, focus size, modulation index and projection angle. 
     
     
       6. A method as claimed in  claim 5  wherein step (a) comprises providing an x-ray tube having a plurality of variable operating parameters which, when varied, modulate a radiation dose, with a modulation speed, for x-ray produced by said x-ray tube, and wherein step (b) comprises operating said x-ray tube while varying multiple operating parameters in said plurality of operating parameters through respective parameter ranges to generate modulation speed data representing modulation speeds respectively for different combinations of said multiple operating parameters, and wherein step (d) comprises varying said multiple operating parameters in advance of said time. 
     
     
       7. A method as claimed in  claim 1  wherein said exposure effect in step (c) is minimization of quantum noise in an x-ray image obtained by said exposure of said subject to said radiation dose, and wherein step (d) comprises varying said operating parameter to produce substantial coincidence between a maximum intensity of said x-rays from said x-ray tube and a peak x-ray absorption of said subject. 
     
     
       8. A method as claimed in  claim 1  wherein step (b) comprises operating said x-ray tube at multiple focus sizes, and varying said operating parameter through said parameter range for each of said multiple focus sizes, and generating said modulation speed data for each of said focus sizes. 
     
     
       9. A method as claimed in  claim 1  wherein step (b) includes obtaining a nominal modulation profile for modulating said radiation dose, said nominal modulation profile having rising portions and falling portions, and employing said modulation speed data to modify only said rising portions of said modulation profile. 
     
     
       10. A computed tomography apparatus comprising: 
       an x-ray tube having at least one variable operating parameter which, when varied, modulates a radiation dose, with a modulation speed, for x-rays produced by said x-ray tube;  
       a radiation detector on which said x-rays are incident;  
       a gantry on which said x-ray tube and said detector are mounted, said gantry being adapted for rotation around a subject to obtain image data representing said x-rays attenuated by said subject;  
       an image computer supplied with said image data for reconstructing an image of said subject therefrom;  
       a dose modulation unit containing modulation speed data obtained while operating said x-ray tube while varying said operating parameter through a parameter range, said modulation speed data representing modulation speeds respectively for different values of said operating parameter; and  
       a controller connected to said x-ray tube and said dose modulation unit for operating said x-ray tube to produce an exposure effect, associated with exposure of a subject to said radiation dose, that is dependent on modulation of said radiation dose by varying said operating parameter in advance of a time at which said exposure effect is to be achieved, according to said modulation speed data, to modulate said radiation dose while irradiating said subject with x-rays from said x-ray tube to produce said exposure effect at said time.  
     
     
       11. A computed tomography apparatus as claimed in  claim 10  wherein said at least one operating parameter is selected from the group consisting of maximum tube current, focus size and modulation index. 
     
     
       12. A computed tomography apparatus as claimed in  claim 10  wherein said x-ray tube has a plurality of variable operating parameters which, when varied, modulate a radiation dose, with a modulation speed, for x-rays produced by said x-ray tube, and wherein said dose modulation unit contains modulation speed data obtained while operating said x-ray tube while varying multiple operating parameters in said plurality of operating parameters through respective parameter ranges, said modulation speed data representing modulation speeds respectively for different combinations of said multiple operating parameters, and wherein said controller varies said multiple operating parameters in advance of said time. 
     
     
       13. A computed tomography apparatus as claimed in  claim 12  wherein said multiple operating parameters are selected from the group consisting of maximum tube current, focus size and modulation index. 
     
     
       14. A computed tomography apparatus as claimed in  claim 10  wherein said gantry rotates said x-ray tube and said radiation detector around said subject to expose said subject to x-rays from said x-ray tube at a plurality of successive projection angles and wherein said at least one variable operating parameter is selected from the group consisting of maximum tube current, focus size, modulation index and projection angle. 
     
     
       15. A computed tomography apparatus as claimed in  claim 14  wherein said x-ray tube has a plurality of variable operating parameters which, when varied, modulate a radiation dose, with a modulation speed, for x-rays produced by said x-ray tube, and wherein said dose modulation unit contains modulation speed data obtained while operating said x-ray tube while varying multiple operating parameters in said plurality of operating parameters through respective parameter ranges, said modulation speed data representing modulation speeds respectively for different combinations of said multiple operating parameters, and wherein said controller varies said multiple operating parameters in advance of said time. 
     
     
       16. A computed tomography apparatus as claimed in  claim 10  wherein said exposure effect is minimization of quantum noise in said image data, and wherein said controller varies said operating parameter to produce substantial coincidence between a maximum intensity of said x-rays from said x-ray tube and a peak x-ray absorption of said subject. 
     
     
       17. A computed tomography apparatus as claimed in  claim 10  wherein said x-ray tube is operable with multiple focus sizes, and wherein said dose modulation unit contains modulation speed data obtained while varying said operating parameter through said parameter range for each of said multiple focus sizes, and said modulation speed data including respective datasets for each of said focus sizes. 
     
     
       18. A computed tomography apparatus as claimed in  claim 10  wherein said controller contains a nominal modulation profile for modulating said radiation dose, said nominal modulation profile having rising portions and falling portions, and wherein said dose modulation unit employs said modulation speed data to modify only said rising portions of said modulation profile.